Hi Micah, output below. However - it looks like the breakage is due to profile issues.
firefox -ProfileManager, and creating a new profile worked. I was able to copy my old extensions directory to the new profile, and all the extensions worked on the new profile. Also tried removing XPC.mfasl, XUL.mfasl etc from my old profile dir. - no luck on having the old profile directory work. So I tried restoring my old profile directory from a backup; trying to launch produced this message: 'Malformed File: Firefox could no install this item because "install.rdf" (provided by the item) is not well-formed or does not exist. Please contact the author about this problem."; after clicking ok, it started with what looked like blank profile.

Can you launch firefox --safe-mode and disable the extensions? Then try enabling one by one if that works. I noticed that you had 2 copies of adblock-plus installed, but I don't know if that would cause the issues you are having.

Micah - Thanks! That launched, kept prefs, bookmarks, etc. With the disabled extensions page, it showed "2 new add-ons have been installed."

-In the dpkg listing, the first adblock-plus listing is a dummy package.
$ dpkg-query -l adblock-plus xul-ext-adblock-plus
...
ii adblock-plus 1.1.3-1 transitional dummy package
ii xul-ext-adblock-plus 1.1.3-1 Advertisement blocking extension for web browsers
... (guess I'll remove the transition package)
I then restarted firefox again (now all extensions off thanks to safe-mode toggle), then enabled extensions; With all the ones I had running prior to 3.6 re-enabled, it's working fine. So I have no idea what/how the profile was corrupted/damaged (or ??) in the 3.6 update (assuming), but it's working now with restore = safe-mode to turn off extensions for first startup... Thanks for your help on getting my firefox running again.

This bug report is being closed due to your last comment regarding this being fixed by tweaking your extensions. For future reference you can manage the status of your own bugs by clicking on the current status in the yellow line and then choosing a new status in the revealed drop down box. You can learn more about bug statuses at https://wiki.ubuntu.com/Bugs/Status . Thank you again for taking the time to report this bug and helping to make Ubuntu better. Please submit any future bugs you may find.

Turns out it's not cleared up, and I don't think it's invalid - looks to be a firefox upgrade bug.http://support.mozilla.com/nl/forum/1/562036?forumId=1
has some other descriptions and people have the issue on OSX, windows, and linux... Clicking restart in the extensions area works with any/all of the previously installed extensions, closing firefox and launching again does not. I suspect profile damage rather than extension incompatibility. For now I'm just running without extensions, but I'll probably create a new profile and see if I can get them to consistently run again.